Dinosaur-Killing Comet Didn't Wipe Out Freshwater Species

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The cosmic impact that ended the age of dinosaurs killed many
living creatures on land and in the sea, but scientists have
found, puzzlingly, that life in freshwater largely escaped this
fate.

Now new research, detailed online July 11 in the Journal of
Geophysical Research-Biogeosciences, suggests freshwater life
survived extinction because they were better adapted to withstand
rapid changes in their surroundings, which helped them outlast
the crises in the wake of the catastrophe.

The mass extinction event the scientists studied (also the most
recent and most familiar) is known as the
K-T event or, more recently, the K-Pg event. The disaster,
which killed off at least 75 percent of all species on Earth,
including all dinosaurs except for birds, was apparently
triggered by a
cosmic impact that occurred in what is now Mexico about 65
million years ago. [ Wipe
Out: History's Most Mysterious Extinctions ]

Past research suggested that while marine life was devastated by
this
mass extinction, freshwater organisms underwent relatively
low extinction rates. Now investigators suggest the secret of
their survival may have been all the variability experienced by
freshwater life.

Gimme shelter

Water would have helped shelter life in rivers and lakes, as well
as the seas and oceans, from the initial blast of heat from the
cosmic impact. However, the giant extraterrestrial collision set
fire to Earth's surface, darkening the sky with dust and ash that
cooled the planet. The resulting "impact winter" and its lack of
sunlight would have crippled both freshwater and marine food
chains by killing off microscopic photosynthetic organisms known
as phytoplankton that are at the
base of the marine and freshwater food chains.

Intriguingly, while marine communities were devastated by the
mass extinction, losing 50 percent of their species, geophysicist
Douglas Robertson at the University of Colorado at Boulder and
his colleagues looked at a database of western North America
fossils and discovered freshwater ones there survived relatively
unscathed, losing only about 10 percent of their species.

The researchers note that freshwater organisms, unlike marine
life, are used to annual freezes that ice over inland waters,
severely limiting their oxygen supplies. As such, freshwater
communities might have better endured the low oxygen levels in
the wake of the death of photosynthetic life following an impact
winter. (Photosynthetic life generates virtually all the oxygen
in the atmosphere, and needs light to live, and the impact winter
would have significantly reduced the amount of sunlight reaching
Earth.)

Impact winter

Inland waters could also benefit from influxes of nutrients from
water seeping in from nearby soils laden with organic matter.
Moreover, such groundwater could also be warm, pumping a welcome
amount of heat into impact-winter-cooled freshwater. In contrast,
while marine coasts might also experience some benefit from warm
groundwater, the vast majority of the ocean would not.

In addition, many freshwater organisms can go dormant, including
eggs or adults buried in the mud. This would have enabled them to
await the return of friendlier conditions, the researchers said.

All these adaptations may have helped freshwater life hold on for
the six months to two years it would have taken until the sky
cleared from the
impact winter. Although the impact event likely killed off
many freshwater organisms as well, "for a species to survive, you
need only a small number of surviving individuals, an absolute
minimum of two individuals at the extreme limit," Robertson told
LiveScience. "Look at what a couple of rabbits were able to do in
Australia in a few decades." Rabbits, first brought to Australia
as food in the 18th century, swarmed uncontrollably across the
continent, once teeming in the billions.

Editor's note: This story was updated to
note where the new research was published.